Generally, a common torque wrench, used for fixing workpieces such as nuts, blots and washers, has no way of knowing whether or not the workpieces are properly tightened except by user's feeling. Since there is no quantitative data provided by the common torque wrench about the force it is exerting, it is impossible for the user to know exactly whether or not the workpiece is already over tightened or is still loose, so that there is always a safety precaution or doubt about the use of those common torque wrenches. Therefore, more and more torque wrench with torque indication are developed, such as torque wrenches with indicator-type or digital display torque meter, or digital power torque wrenches, etc.
There are two types of digital power torque wrenches, which are contact type and non-contact type. The contact type digital power torque wrenches can be exemplified by a power torque wrench disclosed in U.S. Pat. No. 4,544,039, entitled “Torque transducing systems for impact tools and impact tools incorporating such systems”, which is able to obtain and send a torque signal to a gauge by the detection of current using its slip rings and brushes. However, such contact type torque wrench usually has disadvantages such as slow detection, wear- and tear problems, noises, high manufacturing cost, and so on.
For the non-contact type digital power torque wrenches, they can be further divided into two categories which are electromagnetic torque wrenches and optical torque wrenches. The non-contact type electromagnetic torque wrenches can be exemplified by a power torque wrench disclosed in U.S. Pat. No. 5,351,555, entitled “Circularly magnetized non-contact torque sensor and method for measuring torque using the same”, which is operating under the principle that: when the torque wrench is used for tightening a workpiece, the application is going to apply a torque upon its rotating shaft for causing the rotating shaft to deform slightly and thus producing a magnetic field variation in response to the deformation, and then such magnetic field variation is converted by its process control system into a numerical value as an indication of torque which is displayed on its liquid crystal display panel. However, although such non-contact torque wrench has fast detection speed and no wear-and-tear problem, it is still suffered by noise problems and high manufacturing cost.
Therefore, it is in need of a power torque wrench not only can perform a torque measurement in a rapid manner without being troubled by wear-and-tear and noise, but also it is ease to maintain and can be manufactured with comparatively less cost.